The present invention relates to a hydraulic tappet which is installed in a direct acting valve movement mechanism in an internal combustion engine, thereby avoiding a valve clearance automatically in a valve-movement system.
Recently a hydraulic tappet has been provided in a direct-acting valve movement mechanism such as a DOHC-type engine.
A valve clearance is formed with heating during engine driving or with wear of parts in the valve-movement mechanism. A hydraulic tappet makes the valve clearance to zero, thereby decreasing mechanical noise in the valve-movement system. This type of a known hydraulic tappet is disclosed, for example, in Japanese Patent Laid-Open Pub. No. 1-20286. The hydraulic tappet comprises a cylindrical inner casing for guiding a plunger therein, the inner casing being integrally formed with a web which extends in a radial direction inwardly of an outer casing which has a receiving plate which abuts a rotary cam, the receiving plate being fixed on the upper surface of the outer casing.
To increase allowable rotation speed, parts which act in the valve-movement system are made as light as possible, thereby reducing effects involved by inertia force and increasing followability of the valve to a cam. In particular, a hydraulic tappet includes a valve clearance adjusting mechanism to increase the whole weight, which is less advantageous than a normal tappet. It is required to decrease weight thereof. Also, since the hydraulic tappet vigorously moves up and down, high rigidity is greatly required.
In the known hydraulic tappet in which the plunger-containing inner casing is supported only by the web in the outer casing, not only the inner casing but also web or outer casing needs thick-walls and high rigidity. Further the receiving plate which is subject to large load by the cam is fixed on the outer casing. Therefore, to attain high bonding strength by increasing bonding area, the outer casing must be made to have large thickness. If each part has thick walls, the whole weight of the tapped is increased, so that it is impossible to increase allowable rotation speed of an engine.
An object of the present invention is to overcome the foregoing disadvantages and to provide a hydraulic tappet for an internal combustion engine in which the tappet itself is decreased in weight without reducing strength or rigidity, thereby increasing allowable rotation speed.